Old Greek coin, one swears (6)

I believe the answer is:

stater

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'old greek coin' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for stater that I've seen before include "One saying something" , "Old Greek coin" , "One specifying" , "making declaration?" , "Greek currency as earlier" .)

I've seen this clue in the Sydney Morning Herald.

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